The module utilizes the SN76489 chip, a digital sound generator manufactured by Texas Instruments during the early 1980’s. Better known as the “PSG” chip, the SN76 features two square wave channels, a noise channel, and dedicated 4-bit amplifiers. The SN76 possesses a characteristically rich, dark sound, and was used in a number of beloved videogame and computer systems, including the Sega Master System, Sega Game Gear, ColecoVision, BBC Micro, and IBM PCjr.

The OB pairs the simple, nostalgic tones of the SN76 with a deep and innovative control system. Each SN76 channel is paired with two dedicated function generators for controlling pitch and amplitude, for a total of six function generators per preset. In standard mode, the function generators snap to discreet values; in interpolation mode, they smoothly glide between values, generating continuous LFO-like waveforms. A number of standard shapes are provided, but intrepid explorers will find themselves programming custom shapes via the powerful Envelope Graph interface. Combined with their extremely wide speed deltas, the function generators can be used to create anything from short drum sounds and 1-shot effects to slowly moving textures and sequences.

The resulting patches can be saved to any of the OB’s eight preset slots. Presets store virtually every module parameter, and are written to an SD Card for easy management and portability. Each preset is instantly recallable using the Preset CV input or the dedicated gate inputs running along the center of the module. By storing a 1-shot percussive sound in each slot, the eight gate inputs can be used to trigger each preset drum-machine-style, while the preset CV input can be used to sequence or sweep through presets with control voltage. Presets can also be algorithmically generated using the RANDOM button according to three distinct modes: Drum Mode, which generates bass drums, snares, hats, cymbals, and effects; Envelope Mode, which randomizes the function generator shapes; and Sequence Mode, which generates full looping patches and sequences.

Four assignable CV inputs allow for external control over pitch, amplitude, or speed on any SN76 channel. Each input’s assigned routing is stored along with the preset system, allowing for dramatic modulation permutations without physically re-patching the inputs.

Direct CV control over a physical SN76489AN sound chip, with 4-bit amplitude and 1V/Oct-scaled 10-bit pitch